Human bones from a newborn, a child and four adults or teenagers who lived around 40,000 years ago show clear signs of cutting and of fractures to extract the marrow within, they say.

"It is irrefutable, cannibalism was practised here," says Belgian archaeologist Christian Casseyas as he looks inside a cave halfway up a valley in this site in the Ardennes forest.

The bones in Goyet date from when Neanderthals were nearing the end of their time on earth before being replaced by Homo sapiens, with whom they also interbred.

Once regarded as primitive cavemen driven to extinction by smarter modern humans, studies have found that Neanderthals were actually sophisticated beings who took care of the bodies of the deceased and held burial rituals.

Cases of Neanderthal cannibalism have been found until now only in Neanderthal populations in southern Europe in Spain, at El Sidron and Zafarraya, and in France, at Moula-Guercy and Les Pradelles.

The caves at Goyet have been occupied since the Paleolithic era. The 250-metre- (820-feet-) long galleries were dug into the limestone by the Samson, a small stream that still flows a few metres below.

That was until 2004, when the institute's head of anthropology Patrick Semal discovered, hidden in amongst the drawers of what Dupont thought were human bones, a jaw tip that clearly belonged to a Neanderthal.

Scientists have since been painstakingly sorting through fragments that Dupont thought were animal bones to see if there are other traces of ancient man.

- 'Extract the marrow' -

Now an international team led by Helene Rougier, an anthropologist at California State University Northridge in the United States, has proved from the bones found at Goyet that the Neanderthals there were cannibals.

The bones show traces of cutting, "to disarticulate and remove the flesh," said Christian Casseyas, who also leads tours for the public at the caves.

The Neanderthals "broke these bones in the same way that they broke those of the reindeer and horses found at the entrance of the cave, certainly to extract the marrow", he adds.

Rougier, whose work on the Belgian cave was published last July by Scientific Reports, a journal of the Nature group, told AFP that "indeed, we can conclude that some Neanderthals died and were eaten here", which is a first in Northern Europe.

"Some of these bones have also been used to make tools to touch up the edges of flints to re-sharpen them," says Rougier.

But the reasons for the cannibalism remain a mystery, as to the extent to which the Neanderthals ate their dead.

"Was it systematic? Was it only at certain particular moments?" she asks. "I don't know how to interpret the reason behind this cannibalism. It can be purely food, but it can also be symbolic ... The reason remains open," she says.